The invention relates to a slurry hydrocarbon synthesis process in which impurities are removed in situ from a hydrocarbon slurry liquid comprising the raw wax product of the hydrocarbon synthesis reaction. More particularly, this invention relates to independently controlling the temperature at which impurities are removed from the raw wax in an external treatment zone.
Hydrocarbon synthesis (HCS) methods utilizing Fischer-Tropsch processes are well known and described in the art. In a Fischer-Tropsch process, synthesis gas (CO+H2) made, e.g. from natural gas, is converted over a catalyst, e.g. a ruthenium, iron, or cobalt catalyst, to form a wide range of products including gaseous and liquid hydrocarbons and oxygenates and a normally solid high paraffin hydrocarbon wax. Typically, Fischer-Tropsch waxes are upgraded by catalytically converting them to lower boiling paraffinic hydrocarbons falling within the gasoline and middle distillate boiling ranges. This treatment primarily involves hydrogenation, e.g. hydrotreating, hydroisomerization and hydrocracking. However, as new markets continue to expand, demand for high quality waxes has increased. The varied and growing uses for high quality Fischer Tropsch waxes include, e.g., food containers, waxed paper, coating materials, electrical insulators, candles, crayons, markers, cosmetics, etc. Stringent purity requirements are set by regulatory authorities, such as the FDA in the United States and the SCF in the European Union, particularly if the wax is to be used in food and drug applications.
Fischer-Tropsch waxes have many desirable properties. They have high paraffin contents and are essentially free of the sulfur, nitrogen and aromatic impurities found in petroleum waxes. However, untreated raw Fischer-Tropsch waxes may contain a small but significant quantity of olefins and oxygenates (e.g. long chain primary alcohols, acids and esters) formed in the slurry as by products of the HCS reaction. Consequently, there is a need to further treat raw Fischer-Tropsch wax to remove these impurities. This additional treatment is part of a time consuming and costly process as Fischer-Tropsch waxes typically undergo hydroprocessing in order to achieve high purity. These purification measures typically occur in another reactor separate from the reactor where the hydrocarbon synthesis has occurred. A more efficient and direct method of producing purified Fischer-Tropsch wax from a hydrocarbon synthesis process provides for such purification processes to be carried out in situ at the reactor site as disclosed in co-pending United States patent application Ser. No. 09/905,232 filed on even date herewith and incorporated herein by reference. However, while the temperature in the main body of a typical Fischer Tropsch reactor is ideal for the hydrocarbon synthesis reaction which occurs there, various other reactions, including wax purification, may occur optimally at other temperatures. Accordingly, it would be an advance if temperature could be independently regulated for selected reactions carried out in situ simultaneously with the hydrocarbon synthesis reaction.
A preferred mode for operating the Fischer-Tropsch process is a slurry-type process which may be carried out, e.g. in moving bed systems or slurry reactors. The slurry comprises slurry liquid and finally divided catalyst, wherein the catalyst particles are suspended in a liquid hydrocarbon and the CO/hydrogen mixture is forced through the catalyst/hydrocarbon slurry allowing good contact between the CO/hydrogen and the catalyst to initiate and maintain the hydrocarbon synthesis process.
Advantages of a slurry-type process, over that of a fixed bed process are that there is better control of the exothermic heat produced in the Fischer-Tropsch process during the reaction and better control over catalyst activity maintenance by allowing recycle, recovery, and rejuvenation procedures to be implemented. The slurry process can be operated in a batch or in a continuous cycle, and in the continuous cycle, the entire slurry can be circulated in the system allowing for better control of the primary products residence time in the reaction zone.
Slurry reactors are well known for carrying out highly exothermic, three phase slurry-type Fischer-Tropsch reactions. Reactors in which such three phase slurry-type hydrocarbon synthesis processes are carried out are sometimes referred to as xe2x80x9cbubble columnsxe2x80x9d, as is disclosed in U.S. Pat. No. 5,348,982. Bubble column reactors typically have a multiplicity of tubes suspended within a shell type housing, the tubes being filled with a heat transfer medium, e.g. water, which absorbs the heat generated by the exothermic reaction occurring on the shell side of the of the tubes in the main body of the housing. In such a three-phase hydrocarbon synthesis (HCS) process, a synthesis gas comprising a mixture of H2 and CO (syngas) is bubbled up as a third phase through a slurry in the reactor in which the slurry comprises liquid hydrocarbons and dispersed solid particles comprising a suitable Fischer-Tropsch type hydrocarbon synthesis catalyst. The catalyst particles are typically kept dispersed and suspended in the liquid by the lifting action of the syngas bubbling up through the slurry and by hydraulic means. Typically, the slurry liquid is the product of the reaction, usually C5-C100 hydrocarbons. Preferably, the slurry liquid comprises primarily high boiling paraffins (Fischer-Tropsch waxes).
The invention relates to a process for purifying in situ the raw wax product of a slurry type hydrocarbon synthesis (HCS) process in a treatment zone external of the synthesis reaction zone, wherein the reaction temperature in the external treatment zone is controlled independently from the temperature in the synthesis reaction zone. The process involves contacting a synthesis gas comprising a mixture of H2 and CO with a synthesis slurry comprising solid particulate hydrocarbon synthesis catalyst and hydrocarbon liquid in a synthesis zone. Reaction conditions in the synthesis zone include a first reaction temperature effective to form hydrocarbons, at least a portion of which are liquid at said reaction conditions. In the inventive process, the synthesis slurry exits the reactor and is passed into a treatment zone external of the main slurry body. A treatment gas is passed into the treatment zone and contacts the synthesis slurry to at least partially removes impurities therefrom, thus forming a treated slurry. The treatment gas may also optionally act as a lift gas in the treatment zone. The reaction conditions in the treatment zone include a second reaction temperature which is controlled independently from the first reaction temperature in the hydrocarbon synthesis zone. The treated slurry is contacted with wax withdrawal means wherein at least a portion of the purified liquid hydrocarbon wax is separated from the treated slurry. The treated slurry may then be passed back into the synthesis zone from which it was drawn. In a preferred embodiment, off gas produced in the treatment zone is removed from the purified slurry by passing it through gas disengaging and removal means prior to withdrawal of purified wax product.